Those are good points.
It's true, that Google Voice is intended to be used as a call forwarding service, to enhance some other, existing phone service, and not as a standalone phone service. GV originally only forwarded calls to other telephone numbers. Later, forwarding to Google Chat was added, and that is what OBi devices use to receive GV calls, on the OBi's built-in Chat client. Outbound calls are also made via Chat, not GV.
Previously, it was required to maintain a working forwarding phone number on a GV account. When Google introduced Hangouts, it dropped the requirement to maintain a forwarding phone number. Now, calls made to your GV account, which are forwarded either to an OBi via Chat, or to Hangouts, are sufficient to keep a GV number active.
However, there are many benefits to using the service as designed and intended. By adding a forwarding phone, you can remotely manage voicemail messages and text messages, change voicemail greetings, and, of course, receive forwarded calls on the go. If none of that is of interest to you, then you may be better served by an account with a SIP VoIP service provider offering simple telephone service.